Vacuum tube sweep circuit



June 13, 1939. F, JACOB 2,162,335

VACUUM TUBE SWEEP CIRCUIT Filed Feb. 10, 1956 #RE UENC Y TIME INVENTOR FREDERICK IV- JACOB cevflwwyy ATTORNE Patented June 13,1939 i UNITED STATES VACUUM TUBE SWEEP omom'r Frederick N. Jacob, Chicago, 111., assignor to Johnson Laboratories, Inc., Chicago, 111., a corporation of Illinois Application February 10, @936, Serial No. 63,142

Claims. (01. 250-36) This invention relates to improvements in highfrequency apparatus, and more particularly to an improved type of high-frequency oscillator. Oscillators of the type herein contemplated have an 5 output which varies in frequency over a desired range of frequencies, about a selected mean frequency.

An object of this invention is to provide a variable-frequency oscillator which has a substantially constant output of varying frequency.

An additional object of the present invention is 'to provide a variable-frequency oscillator the output of which varies over a desired range which is substantially independent of the mean frequency of the output.

A further object is to provide a variable-frequency oscillator the output of which varies in frequency in only one sense, the output being suppressed during variations in the opposite sense. Still another object of my invention is to provide a variable-frequency oscillator which .is adapted for use with a cathode-ray oscilloscope and which operates without any moving parts or other complicated and expensive apparatus.

To be satisfactory, a variable-frequency oscillator should be so designed that the range of frequency variation is adjustable and substantially unaffected by the value of the mean frequency of the range. Furthermore, adjustment should be provided for altering the mean frequency to a desired value. The output of the oscillator should remainsubstantially constant in amplitude irrespective of the mean frequency and of the range of variation in frequency. Previous variable-frequency oscillators have i usually employed a motor-driven variable capacitor to vary the frequency of the output. This capacitor was connected in shunt with the main oscillator tuning capacitor, and hence the range of frequency variation depended upon the setting of the main oscillator tuning capacitor. This setting'in turn determined the mean frequency of the output, and so the range of frequency variation was dependent upon the mean frequency at which the oscillator was adjusted to operate.

Other difiiculties inherent in said earlier type of variable-frequency .oscillator included maintaining the alignment of the rotating parts, synchronizing the rotation of the motor, and obtaining a desired range of variation in frequency. These and other problems are overcome by the arrangement of the present invention, which employs no moving parts but which provides great flexibility of operation.

of the amplifier, and the output of the amplifier is Although the variable-frequency oscillator of the present invention may be used for numerous purposes, it is especially adapted for operation in conjunction with a cathode-ray oscilloscope for indicating the performance of a resonant system. For example, if it is desired to observe the response of an intermediate-frequency amplifier, the variable-frequency oscillator is connected to the input rectified and supplied to one pair of deflecting 0 plates of the'cathode-ray tube. The other pair of deflecting plates is supplied with a saw-tooth voltage which is also utilized to synchronize the sweep of the oscillator. 16

.My invention will be better understood if the following description is read in conjunction with the accompanying drawing, in which: f

Fig. 1 is a schematic diagram of one embodiment of the invention together with associated apparatus; and 20 Fig. 2 is a graph showing how the frequency of the output of the embodiment of Fig. 1 varies with time.

Instead of employing a single oscillator circuit, the resonant frequency of which is varied both in selecting the mean frequency and in producing the frequency sweep, the present invention contemplates the use of two separate and distinct oscillators. I The frequency of the main oscillator is adjusted to obtain the desired mean frequency and the frequency of the auxiliary oscillator is varied to produce the desired frequency sweep, the

.outputs of the two oscillators being combined to obtain a single potential having a desired mean frequency and a desired periodic variation of its 35 frequency. By employing such an arrangement, I am enabled to alter either the mean frequency or the amount of sweep with no appreciable effect of either upon the other.

The use of two oscillators has the additional advantage of greatly extending the range of mean frequencies which may be covered with a given set of inductors "and tuning capacitors. This feature simplifies the construction of the device and greatly increases its usefulness. The frequency range maybe still further extended by employing tapped inductors in one or both oscillator circuits.

Referring to Fig. 1, the main oscillator in- 50 eludes vacuum tube l, inductor 2, tuning capacitor 3 shunted by trimmer capacitor 4, and fixed capacitor 5. Resistors Ii and I in series are shunted across portion 8 of inductor 2, and the free end of resistor I is grounded. Plate poten- 55 tial is supplied to vacuum tube I through choke coil 9 and filter resistor ill from a tap on a source of direct-current potential il. Filter capacitor i2 supplements resistor l0. Vacuum tube l is shown as a triode, but it will be understood that other types of tubes, as well as different oscillator circuit arrangements, may be employed without departing from the scope of the present invention. The structure described constitutes a first oscillation generating means tunable over a predetermined range by operation of the tuning capacitor 3.

Vacuum tube I3 is of the pentagrid-converter type. Its cathode I4 is grounded through bias resistor l5 shunted by capacitor l6. Grid I1 is connected to cathode l4 through grid leak II. The ungrounded terminal of inductor I9 is connected to grid I! through grid capacitor 29. Inductor i9 is shunted by variable capacitor 2| and, when switch 22 is in the position shown, by adjustable capacitor 23. These parts connected as described, constitute a second oscillation generating means normally developing oscillations at a predetermined frequency established by the adjustment of the adjustable capacitors 2| and 23 when the switch 22 is in the position shown in Fig. 1.

Grid 24 of vacuum tube I3 is connected to the high-potential terminal of inductor 25, the lowpotential end of which is connected through resistor 26 and 21 to a terminal of choke 26. A filter capacitor 29 is connected between the junction of resistors 26 and 21 and ground. Inductor 25 is inductively coupled to inductor i9, so that the elements l4, I1 and 24 of vacuum tube 13 function as a vacuum-tube oscillator.

Grid 36 of vacuum tube I3 is connected to the junction of resistors 6 and I. Screen-grid 3| of vacuum tube i3 is connected to the junction of voltage-divider resistors 32 and 33, and bypassed to ground by capacitor 34. The free end oi resistor 33 is connected to the junction of resistor 21 and choke 26. Plate 35 of vacuum tube I3 is connected to the high-potential terminal of inductor 36, which is shunted by adjustable capacitor 31. The remaining terminal of inductor 36 connects to the junction of resistor 21 and choke 26. The plate circuit of vacuum tube i3 is tuned by means of variable capacitor 36, which may be operated in gang with capacitor 3.

Gaseous discharge tube 39 is supplied with grid and plate potentials from direct-current source 40. The potential on grid 4| is adjusted by means of variable resistor 42 which is connected between cathode 43 and the negative terminal of source 46. Plate 44 of tube 39 is'connected through inductor 45 and limiting resistor 46 to the positive terminal of source 40, which is by-passed to cathode 43 of tube 39 by means of capacitor 41. Plate 44 of tube 39 is also connected to grid 24 of vacuum tube I! through capacitor 61. Adjustable tap III on source 40 is grounded.

Cathode 43 is connected through resistor 46 to the high-potential terminal of potentiometer 4|. Horizontal sweep voltage may be tapped of! at terminals 66 and 69, connected respectively to cathode 43 and to ground. Movable arm 52 of potentiometer 49 is connected through resistor 53 to grid 64 of vacuum tube 55. Cathode 56 of vacuum tube 55 is grounded through resistor 51 which is shunted by capacitor 56. Plate 69 of vacuum tube 55 is connected through inductor 46 to the junction of resistor 21 and choke 26. Resistor 4|v is connected between cathode 56 and the junction of inductor 66 and choke 26, and this junction is by-passed to ground by capacitor 62. Capacitor 63 is connected between grid 54 and plate 59 of vacuum tube 55. Grid 54 is connected through capacitor 64 to terminal 65 of switch 22.

In operation, vacuum tube l operates as the main oscillator, and its frequency of oscillation is determined by means of variable capacitor 3. A portion of the output of oscillating vacuum tube i, which appears across resistors 6 and 1 in series, is supplied to the control-grid 36 of mixer vacuum tube 13. Interaction due to the common source of direct-current potential H is prevented by choke 9, series resistor l6 and capacitor i2.

The elements I4, I! and 24 of vacuum tube I 3 form the auxiliary oscillator, the oscillatory frequency of which depends upon the amount of capacitance in shunt with inductor l9. This capacitance consists of variable capacitor 2| and either adjustable capacitor 23 or the reflected capacitance of vacuum tube 55 in series with capacitor 64, according to the position of switch 22.

Vacuum tube l3 combines the outputs of the main and auxiliary oscillators, and its plate current has a beat-frequency component. Inductor 36 is tuned, not to resononce with this component but well below the beat frequency, by means of adjustable capacitor 31 and variable capacitor 39. If desired, the latter capacitor may be operated in gang with variable capacitor 3 to facilitate a shift in the mean frequency of the output which appears across inductor 66 inductively coupled to inductor 36.

Vacuum tube 55 is provided with an inductively reactive load, so that changes in the directcurrent voltage applied to control-grid 54 produce a substantial variation in the reflected gridto-cathode capacitance. The amount of variation is increased by means of capacitor 63 connected between the grid 54 and plate 59 of vacuum tube 55. By throwing switch 22 to contact 65, the input capacitance of vacuum tube 55, in series with capacitor 64, is placed in shunt with variable capacitor 2!. Hence a. periodic change in the input capacitance of vacuum tube 55 will produce a corresponding variation in the oscillatory frequency of the auxiliary oscillator. When the switch 22 is on contact 65, the vacuum tube 55 operating as described, constitutes an auxiliary means for effecting cyclic deviations of the oscillations of said second oscillation generating means, from the predetermined frequency resulting for this position of the switch 22, from the adjustment of the adjustable capacitor 2|.

Gaseous tube 39 causes the periodic discharge of capacitor 41, which in turn produces a change in the potential across resistor 48 and potentiometer 49 in series. During discharge, tube 39 has a very low impedance between plate 44 and cathode 43, and therefore effectually grounds grid 24 of vacuum tube l3 for high frequencies. In this way, the output of the system is practically zero during the discharge of capacitor 41.

A desired portion of the potential across potentiometer 49 is tapped off by means of slider 52 and supplied to grid 54 of vacuum tube 55 through resistor 53. Since one terminal of potentiometer 49 is grounded, a potential of saw-tooth character is applied to grid 54 in addition to the initial bias voltage which is developed across resistor 51. The potentiometer 49 and slider 52 connected as described with the grid 54 of the vacuum tube 55, constitute a means for continuously adjusting the cathode-ray tube.

' amaaas effectiveness of the auxiliary means for effecting cyclic deviations of the oscillations of said second oscillation generating means, to adjust the range of said deviations without affecting the predetermined frequency of the oscillations of said second oscillation generating means.

The frequency at which the capacitor 41 charges and discharges depends mainly upon the constants of the circuit of gaseous discharge tube 39. A synchronizing voltage may be applied to grid 4| to maintain an exact rate of operation if desired. For example, a small voltage derived from the. commercial power supply may be employed to maintain the frequency of operation substantially constant. The saw-tooth voltage developed across resistor 48 and potentiometer 49 in series may be applied to the horizontal defleeting plates of a cathode-ray tube, by connecting them to terminals 68 and 69. In this case, the fact that both the horizontal sweep voltage and the frequency-modulated voltage which is ultimately applied to the vertical deflecting plates of the cathode-ray tube, are regulated from the sam point, insures perfect synchronization and very satisfactory results.

Fig. 2 shows how the frequency of the potential which is developed across inductor 66 varies with time. The frequency gradually increases as capacitor 41 of Fig. 1 is charged, and then suddenly drops back to its initial value when the gaseous discharge tube 39 of Fig. l ionizes. By so arranging the circuit that the ionization of tube 39 causes the output of the auxiliary oscillator to be shunted to ground through capacitor 61, the beatfrequency voltage across inductor 66 becomes substantially zero during the time when the frequency is decreasing, thus causing theueturn trace to serve as a base line on the screen of the This is an advantageous feature of the present invention, and greatly simplifies the use of a cathode-ray oscilloscope.

The operation of my invention, for example in conjunction with a cathode-ray oscilloscope, may be better understood from a description of the adjustment and performance of an embodiment according to Fig. 1 for a particular case. Let it be assumed that it is desired to produce an alterhating-current voltage having a mean frequency of 500 kc. and varying in frequency from 10 kc. below this mean value to 10 kc. above the, mean value, this frequency sweep to take place 60 times per second, and to consist of a relatively slow increase from 490 to 510 kc., with a relatively very rapid return from 510 kc. to 490 kc., during which the voltage is to be substantially zero. Let it also be assumed that it is desired to produce'a directcurrent voltage which increases and decreases synchronously with the increase and decrease in the frequency of the alternating-current voltage.

By means of variable capacitor 3, the main oscillator, consisting of vacuum tube I, inductor 2 and capacitors 3 and 4, isadjusted to produce a frequency of approximately 1500 kc. Movable arm 52 is placed at the grounded end of potenti- 05 ometer 49, and switch 22 is thrown to contact 65. The frequency of the auxiliary oscillator consisting of cathode l4 and grids l1 and 24 of vacuum tube l3, inductors l9 and 25 and capacitor 2|, is now adjusted to produce a frequency of 2000 kc. by means of variable capacitor 2|. A voltage will now appear at the terminals of the inductor 66, this voltage being the beat between the oscillations generated by the two oscillators and therefore having a frequency of 500 kc., which is the mean value of frequency of the desired voltage.

its initial value.

Movable arm 52 on potentiometer 49 is now adquency sweep of 20 kc. The operation of gaseous discharge tube 39 is such that capacitor 41 is charged at a relatively slow rate, and as its charge increases the movable arm 52 on potentiometer 49 becomes less positive or increasingly negative with respect to ground, according to its initial potential. This, in turn, increases the negative potential on grid 54 ofvacuum tube 55, and produces a decrease in its effective input capacitance. Since this-input capacitance is part of the capacitance in the tuned circuit of the auxiliary oscillator, the frequency of this oscillator will now increase from its initial value to some higher value. The initial potential with respect to ground of the movable arm 52 on potentiometer 49 and hence the value of the input capacitance of vacuum tube 55, is determined, for any given adjustment of potentiometer 49, by the setting of adjustable tap 10 on source 40, and the bias voltage appearing across cathode resistor 51. Initially tap 19 is so set that the potentialappearing across terminals 69 and 69 varies from a given amount positive with respect to ground as condenser 41 begins to charge, to an equal amount negative with respect to ground as condenser 41 charges up sufliciently to cause tube 39 to ionize. Any setting of the arm 52 of potentiometer 49 then produces a potential from the arm 52 to ground which varies equally positive and negative. In the assumed case, arm 52 is so set on potentiometer 49 that.

the maximum value of said input capacitance corresponds with an auxiliary oscillator frequency of 1990 kc. Thus the frequency of the auxiliary oscillator is increased from 1990 kc. to 2010 kc., and the frequency of the beat between the outputs of the main and the auxiliary oscillators is increased from 490 kc. to 510 kc.

When capacitor 41 has reached a critical voltage, gaseous discharge tube 39 becomes conductive and discharges capacitor 41 at a very rapid rate. The discharge of capacitor 41 brings the current through resistor 48 and potentiometer 49 back to its initial value and this, in turn, brings the voltage on the grid of vacuum tube back to The input capacitance of this tube, therefore, returns to its initial value and this in turn brings the frequency of the auxiliary oscillator back to 1990 kc.

The operation of gaseous discharge tube 39 is such that the relatively slow charging of capacitor 41' may, for example, occupy ths of the time period and the relatively very rapid discharge of capacitor 41 only th of the time period. The complete time period for the charge and discharge of capacitor 41 is adjusted to be th second by means of variable resistor 42. Thus the time during which the frequency of the auxiliary oscillator is increasing from 1990 kc. to 2010 kc. is ths of a second, while the time period during which it is decreasing from 2010 to 1990 kc. is th of a second. Moreover, by means of the .connection through a suitable direct-current amplifier to the horizontal deflecting plates of the cathoderay' tube, a visual trace of the resonance characteristic of the alternating-current amplifier may be obtained, together with a horizontal base line.

To alter the mean frequency of the alternating-current voltage without altering the width of the band over which the frequency of this voltage is swept, it is only necessary to adjust the frequency of the main oscillator by adjustment of variable capacitor 3. This changes the frequency of the beat between the outputs of the main oscillator and the auxiliary oscillator, but since the frequency of the auxiliary oscillator remains unchanged, the band width over which this frequency is swept by the action of vacuum tube 55 and gaseous discharge tube 39 also remains unchanged.

To alter the width of the band over which the frequency of the alternating-current voltage is swept without altering the mean frequency of this voltage, the adjustment of arm 52 of potentiometer 49 is changed to secure the desired band width. Tap III is utilized only for centering the frequency sweep about a desired mean frequency, and has no appreciable effect upon the j width of the band over which the frequency-is swept.

In adjusting an amplifier under test for initial resonance, it is convenient to adjust capacitor 23 to the mean value of the capacitance resulting from the action of vacuum tube 55 and gaseous discharge tube 3!. By throwing switch 22 to the position shown in Fig. l of the drawing, capacitor 23 is substituted for the mean input capacitance of vacuum tube 55, and the beat frequency between the outputs of the main and auxiliary oscillators becomes constant.

It will be understood that in place of batteries II and ll there may be substituted a suitable rectifier and voltage divider system for operation from the alternating-current supply mains. It will also be understood that other forms of oscillator circuits and other means for mixing the outputs of two oscillators may be employed without departing from the scope of the present invention. Furthermore, the invention is not limited to the uses and applications suggested herein, but may be employed in numerous other ways which will occur to those skilled in the art.

Having thus described by invention, what I claim is:

1. A generator of high-frequency oscillations of periodically varying frequency including a first oscillator, a second oscillator having inductance and capacitance, means for combining the outputs of said oscillators to produce a beat-frequency voltage, a vacuum tube having input capacitance and having a control electrode, a cathode and an output electrode, said input capacitance being connected effectively in shunt with the capacitance of said second oscillator, a

resistor operatively connected between said control electrode and said cathode, and a gaseous discharge tube having a capacitor in its output circuit and arranged so that the periodic discharge of said capacitor produces corresponding changes in the current flowing through said resistor whereby said input capacitance is periodically varied.

2. A generator of high-frequency oscillations of periodically varying frequency including first and second oscillators, means for combining the outputs of said oscillators to produce a beatfrequency voltage, manual means for adjusting the frequency of said first oscillator to a desired value, means for periodically varying the frequency of said second oscillator over a range, and means including a variable voltage producing means for manually adjusting the width of said range substantially independently of the mean frequency of said beat-frequenc voltage.

.3. In combination, a manuall adjustable highfrequency oscillator, a second oscillator, means for automatically varying the frequency of said second oscillator between limits including an arrangement for producing a periodically varying direct-current voltage, means for-combining the outputs of said oscillators to produce a beat-frequency voltage varying in frequency, means for suppressing said beat-frequency voltage during a portion of each cycle of said frequency variation, and means for manually varying the limits of said frequency variation substantially independently of the mean frequency of said beat-frequency voltage.

4. In combination, a manually adjustable high-frequency oscillator, a second oscillator having inductance and capacitance, means for automatically varying the frequency of said second oscillator'between limits, said means includ-- ing a vacuum tube having input capacitance and having a control electrode, a cathode and an output electrode, said input capacitance being connected effectively in shunt with the capacitance of said second oscillator, means for combining the outputs of said oscillators to produce a beatfrequency voltage varying in frequency, means for producing a direct-current voltage varying synchronously with said frequency variation, means for applying said-direct-current voltage to said control electrode, means for suppressing said beat-frequency voltage during a portion of each cycle of said frequency variation, and means for manually varying the limits of said frequency variation substantially independently of the mean frequency of said beat-frequency voltage.

5. In combination, a manually adjustable high frequency oscillator, a second oscillator having inductance and capacitance, means for producing a periodically varying direct current voltage, means including a vacuum tube having input capacitance and a control electrode for automatically varying the frequency of said second oscillator between limits, said input capacitance being connected in shunt with the capacitance of said second oscillator, means for combining the outputs of said two oscillators to produce a beat frequency voltage, means for applying said direct current voltage to said control electrode to periodically vary said beat frequency between limits, and means for automatically maintaining said beat frequency voltage at substantially constant amplitude independent of the mean frequency of said beat frequency voltage.

6. In combination, a manually adjustable high frequency oscillator, a second oscillator having inductance and capacitance, means for producing a periodically varying direct current voltage, means including a vacuum'tube having input capacitance and a control electrode for automatically varying the frequency of said second oscillator between limits, said input capacitance being connected in shunt with the capacitance of said second oscillator, means for combining the outputs of said two oscillators to produce a beat frequency voltage, means for applying said direct current voltage to said control electrode .to periodically vary said beat frequency .between limits, means for suppressing said beat frequency voltage during a portion of each cycle of said frequency variation, and means for automatically maintaining said beat frequency voltage at substantially constant amplitude during the remainder of each said cycle independent of the mean frequency of said beat frequency voltage.

'7. The method of generating a high-frequency alternating potential manually adjustable as to its mean frequency and having a cyclic variation in frequency between limits which are manually adjustable to include a desired range and which are independent of said mean frequency, which comprises producing first and second alternating currents differing in frequency by said mean frequency, combining said alternating currents to produce saidpotential, manually adjusting the frequency of said first alternating current, producing a periodically varying voltage, applying said voltage to vary-the frequency of said second alternating current over a range, and manually adjusting said voltage to vary the width of said range to a desired value while maintaining said mean frequency substantially constant.

8. The method of generating a high-frequency alternating potential having a cyclic variation in frequency between limits manually adjustable to include a desired range and independent of the mean frequency of said potential, which comprises producing flrst and second alternating currents differing in frequency by said mean frequency, producing a direct-current potential whose magnitude varies periodically at the frequency of the desired cyclic variation, applying said direct-current potential to produce a corresponding reactance variation, applying said rcactance variation to produce a corresponding variation in the frequency of said first alternating current, and combining the resulting frequencyvarying current with said second alternating current.

9. An oscillation generator including means for no rmally developing oscillations at a predetermined frequency, auxiliary means for effecting cyclic deviations of said oscillations from said predetermined frequency, and means for continuously adjusting the effectiveness of said auxiliary means to adjust the range of said deviations without affecting said predetermined frequency.

10. An oscillation generator including an oscillation circuit for normally developing oscillations at a predetermined frequency, auxiliary means coupled to said oscillation circuit to shift the resonant frequency thereof cyclically, thereby to effect cyclic deviations of said oscillations from said predetermined frequency, and means for continuously adjusting the effectiveness of said aux iliary means on said oscillation circuit to adjust the range of said deviations without affecting said predetermined frequency.

FREDERICK N. JACOB. 

